Early-onset periodontitis is a disease characterized by a progressive loss of the tooth supporting tissue in adolescents and young adults. This study was undertaken to study the pattern of progression of this disease and to identify potential risk factors. Adolescents with early-onset periodontitis were identified within a population of 14,000 pupils in grades 8 to 12 examined by a national survey of the oral health of US children conducted by NIDR during the 1986/87 school year. From the same national survey two controls per case matched by age, gender, race and geographic location were also located and invited to participate in the study. During 1993 examinations were made on approximately 265 of these young people 19-25 years of age identified from the earlier survey. It has been suggested that specific microorganisms are important in the pathogenesis of early-onset periodontitis. However, this is still a controversial issue since most published data are based on cross-sectional studies and there are only few longitudinal studies of early-onset periodontitis. There is ample evidence showing that the host immune system plays a significant role in the pathogenesis of periodontal diseases. However, it is clear that the mechanisms involved in this process are very complicated, and a better understanding of these mechanisms may help in the management of these diseases. Early-onset periodontitis represent a suitable model for studying the significance of the immune system in the destruction of periodontal tissues. This year data analysis continued, and the following are the main findings: (1) The results suggest a significant association between gingival inflammation and subgingival calculus and the development and progression of early-onset periodontitis. (2) Several bacterial species were associated with early onset periodontitis including Porphyromonas gingivalis, Prevotella intermedia, Fusobacterium nucleatum, Campylobacter rectus and Treponema denticola. However, P. gingivalis and T. denticola were the most significant species and we hypothesize that these 2 species play a significant role in the more severe and progressive forms of this disease. Actinobacillus actinomycetemcomitans was not significantly associated with early onset periodontitis, though it was recovered more often from subjects with localized early onset periodontitis. Eikenella corrodens was present equally in subjects with and without disease. (3) There was a significant increase in beta-glucuronidase activity in gingival fluid with the increase in periodontal probing depth. Sites with gingival bleeding on probing had a significantly higher beta-glucuronidase activity than sites without bleeding. Sites harboring high levels of one or more of 7 bacterial species tended to have high beta-glucuronidase activity. There were moderate differences between the organisms with respect to their effect on beta-glucuronidase activity, but sites with high numbers of P. gingivalis, P. intermedia, or T. denticola also had the highest beta-glucuronidase activity.